The present invention relates to a method for equalising the orientation of fillers and/or distribution of fillers in a moulding compound comprising filled plastic material of an injection moulded part, in particular for making the fibre orientation isotropic in a fibre-reinforced moulding compound. The reinforcing materials can comprise for example glass, carbon, ceramics, aramide or other materials. There should be understood by the term “fibre reinforcement” also materials with fibre-like, oblong or also plate-like shape. It concerns furthermore a corresponding injection moulding method, uses of these methods and also corresponding injection moulded parts. Injection moulded parts of this type are used for example as components in the automobile industry. There they replace previously metallic components since they have an advantage relative to these not only because of their low specific weight but also, by introducing various components, their mechanical and thermal properties can be correspondingly adapted to the respective loading of the component.
Filled plastic materials are used for components of this type. The moulding compound thereby contains a thermoplastic and also a filler. As filler, fillers of any shape, for example oblong or spherical particles, can be used.
An important example of plastic materials filled in this way are fibre-reinforced moulding compounds which contain a thermoplastic and are reinforced by fibres, in particular glass fibres.
If moulding compounds of this type are injected in the molten state into an injection moulding tool, then the result during filling of the cavity by the melt flow is an orientation of the fillers and/or unequal distribution of the fillers within the plastic material matrix. Strong shear and extensional flows which cause an orientation of the filler particles or separation inter alia are responsible. This leads to anisotropic properties of the component with respect for example to its strength, rigidity, shrinkage or even thermal conductivity and expansion. The anisotropic shrinkage is in turn the main cause of component distortion, as is found frequently in fibre-reinforced injection moulded parts. In the flow shadow of flow impediments, a particularly pronounced anisotropy occurs. Joint lines, as are unavoidable in a multiplicity of moulds in injection moulding technology, in fact form fibre orientations parallel to the course of the joint line so that these regions, during stressing perpendicular to the joint line, have strength merely of the order of magnitude of the non-reinforced or non-filled matrix material.
Further problems which are produced by the rheological orientation mechanisms of the injection moulding melt are known with plate-like reinforcing materials or effect pigments. For example, mineral reinforcements are inclined to migrate in regions of great shear orientation towards the component surface. This leads to optical impairment of the component. In the case of injection moulding materials and in particular in the case of transparent thermoplastics which are doped with plate-like pigments, e.g. metallic micas, the rheological orientation of the pigment particles is found to be particularly disruptive in the joint line regions or regions of pronounced shear- and extensional flows. The preferred orientation of the pigments with respect to the surface is disrupted and a significant impairment in the optically homogeneous appearance occurs. Disruptions of this type in the optical appearance are for example described in EP 0 994 915 B1 or also in U.S. Pat. No. 6,194,507 B 1, the solution to this problem being however sought constantly in an improved mixing of pigment and plastic material matrix.
These problems are already known in particular for fibre-reinforced plastic materials and various methods are revealed in the state of the art with which an isotropic fibre orientation is striven for. Thus this problem is mentioned for example in “Kunststoffe” November 2004, pages 72 to 74. In “Plastverarbeiter” 2003, volume 54, no. 6, page 38 to 39, it is proposed in order to solve this problem to place the joint lines in less critical regions of a component. This publication proposes as a further possible improvement to use counter-cycle injection moulding or the Scorim method. As a further possibility, the cavity can be filled over a plurality of sections with temporally delayed opening in a so-called cascade arrangement. Further alternatives are revealed, such as for example setting a different dwell pressure on both sides of a flow line. As a result of the consequently produced different shrinkage on both sides of the flow line, a non-orthogonal orientation of the fibres is sought.
Furthermore, the equalisation of the orientation of polymer molecule chains in the melt state by ultrasound during injection moulding of an optical disc made of polycarbonate, however without the presence of fillers, is known from JP 62249717A.
However none of the methods mentioned here is suitable for making the fibres satisfactorily and adequately isotropic and hence effecting a sufficient strength increase in the injection moulded part.
Similar problems are also known in the case of pigment-filled plastic materials, e.g. with effect pigments, such as metallic micas or aluminium flakes in which, because of unequal distribution and orientation of the pigments, joint lines and flow lines remain visible in injection moulded parts comprising a transparent plastic material. This is known for example from mirror shells (rear mirror housings) in the automobile industry.
Because of high shear forces which can occur close to the sprue, migration of mineral reinforcements was already observed at the sprue point of injection moulded parts.
It is therefore the object of the present invention to make available a method for equalising the orientation of fillers and/or distribution of fillers in an injection moulded part or for increasing the strength of an injection moulded part.
This object is achieved by the method according to claim 1. Advantageous developments of the method are indicated in the dependent claims.